Fix remove early initialization of res.{partner,company,users}

[azoellner]

This early initialization was there to prevent exceptions caused by fetching values of (newly added) fields of those models that were not yet present in the database, but had already been added to the model on the Python side.

Unfortunately, this did run before any module migration. Consequently, a module's pre-migration, which should run before any module and module upgrade, did then run only after those models had already been updated.

Also, the early initialization as implemented did already come too late. It had to run before check_auto_install(), but did run only in update_changed(), a couple of code lines later.

In the end, it is unnecessary if prefetch_fields=False is in the context.

But the context_get() does not put this entry into the context.

This commit fixes all those issue. It makes the early initialization of the three models superfluous, and then removing it automatically let's the migration work as expected again.

[azoellner]

Hint: Probably the code of OCA/server-tools#3487 needs to be adapted accordingly as well:

-             ctx = (
-                 odoo.api.Environment(cr, uid, {})["res.users"]
-                 .with_context(prefetch_fields=False)
-                 .context_get()
-             )
+             ctx_prefetch = {"prefetch_fields": False}
+             ctx = odoo.api.Environment(cr, uid, ctx_prefetch)["res.users"].context_get()
+             ctx = dict(ctx)
+             ctx.update(ctx_prefetch)